The present invention relates to a process for oxyacylation of olefins or diolefins. In particular, the present invention is directed to a process for the production of vinyl acetate from ethylene, acetic acid and an oxygen-containing gas in the presence of a catalyst. More particular, the present invention is directed to a process for the production of vinyl acetate wherein both a liquid and vapor phase are present. Related Information
Both liquid and gas phase reaction are known for the reaction. The basic chemical reaction is: 
The liquid type usually requires a chloride salt while the gas type is chlorine free. The operational conditions for both processes are similar. In the liquid phase process, in the presence of palladium salts and redox systems, vinyl acetate and acetaldehyde are formed side by side. However, in the gas-phase process using palladium or chlorine-free palladium salts, vinyl acetate is almost exclusively formed. Furthermore, no noticeable corrosion problems arise with the gas-phase process.
Some liquid processes use a supported palladium acetate catalyst. The metal and salt catalysts contain alkali acetates and other components which serve to increase activity and selectivity. During the course of the reaction, the catalyst goes through a change, especially in relation to its alkali acetate content. The alkali acetates migrate from the catalyst as a consequence of the reaction conditions and must therefore be constantly renewed. The vapor-phase process differs from the liquid in that very little acetaldehyde forms. A gaseous mixture of acetic acid, ethylene, and oxygen is blown over the catalyst in a tubular reactor and the exit stream, containing vinyl acetate, unreacted starting materials, water, and small amounts of acetaldehyde, carbon dioxide, and other by-products, is separated by a combination of scrubbers and distillation stages.
The commercial production of vinyl acetate by reacting ethylene, acetic acid and oxygen together in the gas phase in the presence of a fixed bed catalyst containing palladium, a promoter metal, and an alkali metal acetate is known. Usually the fixed bed catalyst components are supported on a porous carrier such as silica, zirconia or alumina. There are various patents such U.S. Pat. No. 3,759,839 and Great Britain Patent 1,266,623 which disclose the manufacture of vinyl acetate utilizing palladium promoted catalyst. In a typical vinyl acetate production process, ethylene, acetic acid and oxygen are introduced into a reactor via an inlet. The reactants are contacted with a palladium-containing catalyst and react to produce an outlet stream which is removed from the reactor and cooled. Vinyl acetate, water and the unreacted acetic acid in the outlet stream are condensed and separated for finer purification. The remaining gaseous components of the outlet stream (e.g., ethylene) are compressed and recycled.
Currently, the preferred route to vinyl acetate is the direct reaction of ethylene, acetic acid and oxygen to produce vinyl acetate, water and byproducts. The preferred version of this process uses a heterogeneous catalyst and is performed in the vapor phase at 2-5 bar at 300xc2x0 F. Because of the explosion hazards associated with this reaction, the reaction must be performed with less than a stoichiometric amount of oxygen; hence, conversions of ethylene, acetic acid and oxygen are typically 10-15%, 15-30% and 60-90% respectively. About 5-10% of the ethylene is converted to carbon dioxide and about 1% is converted to acetaldehyde. The low ethylene and acetic acid conversions per pass require extensive recycling along with a carbon dioxide removal system. Although the capital costs of an ethylene-acetic acid-oxygen-based vinyl acetate plant are high, these capital costs are offset by the generally low costs of ethylene and acetic acid. Thus, a need exists for a process for preparing vinyl acetate having higher conversions per pass and lower yield loss to carbon dioxide than the ethylene-acetic acid-oxygen-based route. The process of the present invention, unlike the ethylene-acetic acid-oxygen-based route, indeed produces vinyl acetate in high conversions per pass and does not produce significant quantities of carbon dioxide.
Other attempts at producing vinyl acetate have also been tried. For example, a number of these attempts seek to prepare vinyl acetate from mixtures of carbon monoxide and hydrogen (synthesis gas) because of the very low cost of raw materials. As initial steps, these schemes convert synthesis gas to methanol or dimethyl ether. In addition, many combinations have been tried in which methyl acetate (produced from methanol and recycled acetic acid) or dimethyl ether are carbonylated to produce acetic anhydride. In some schemes, acetic anhydride is partially hydrogenated to produce EDA and acetic acid. In still other schemes, the methyl acetate or dimethyl ether is carbonylated in the presence of hydrogen to produce EDA and acetic acid in one step. Variations on this approach include reacting methanol or methyl acetate with hydrogen and carbon monoxide to produce acetaldehyde and water or acetaldehyde and acetic acid, respectively; however, the selectivity to acetaldehyde in these reactions is poor. The resulting acetaldehyde is then reacted with the acetic anhydride to produce EDA.
It is known to produce vinyl acetate by reaction of ethylene, oxygen and acetic acid using a catalyst comprising a palladium group metal and/or a compound thereof, gold and/or a compound thereof, and copper, nickel, cobalt, iron, manganese, lead or silver, or a compound thereof, preferably deposited on a support material. U.S. Pat. No. 5,332,710, U.S. Pat. No. 5,347,046 and U.S. Pat. No. 5,567,839, among others, disclose methods of producing catalysts of this type suitable for the present invention.
An advantage of the present mixed phase process is over the gas or liquid phase alone. Because of the presence of a liquid phase, the temperature in the reactor is better controlled by allowing a portion of the liquid to form more boil up.
The present invention is a process for producing vinyl acetate comprising reacting ethylene, acetic acid and oxygen together in at least partially liquid phase in the presence of a catalyst comprising a noble metal component by:
(A). concurrently passing said ethylene, acetic acid and oxygen together in concurrent flow through a reaction zone containing said catalyst to produce an effluent containing vinyl acetate under conditions of temperature and pressure such that the temperature of the effluent is above its boiling point and below its dew point, whereby at least a portion but less than all of the material in said reaction zone is in the vapor phase, preferably the flow is downflow or
(B). a. feeding ethylene, acetic acid and oxygen to a reaction distillation zone;
b. concurrently in said reaction distillation zone:
i. contacting ethylene, acetic acid and oxygen in the presence of said catalyst to produce a reaction mixture containing vinyl acetate and
ii. fractionating said reaction mixture into an overheads containing vinyl acetate and a bottoms containing acetic acid and water, the catalyst may be prepared as distillation structure.
The reaction as conducted is strongly exothermic and is carried out suitably at 150-300xc2x0 F. and 2-5 bar. The explosion limit determines the O2 content in the feed mixture which results in ethylene conversion (approximately 10%) in conventional vapor or liquid-phase processes. Oxygen concentrations on the order of 10 to 20 vol.% or higher are contemplated for use in the present process.